142 LECTURE IX. 



in concluding that these phenomena belong to the same 

 function, that the formation of starch is intimately connected 

 with the decomposition of carbon dioxide. Sachs' conclusion 

 was fully confirmed by Godlewski, who found that if leaves 

 exposed to light are not supplied with carbon dioxide no 

 starch-grains are found in their chlorophyll-corpuscles. The 

 explanation of the increase of weight observed by de 

 Saussure is then this, that the decomposition of carbon 

 dioxide by the green parts of plants is connected with the 

 formation of organic substance, and that starch is the visible 

 product of the constructive processes. 



But is the starch which makes its appearance in chloro- 

 phyll-corpuscles under the influence of light to be directly 

 connected with the decomposition of carbon dioxide which 

 goes on in them ? It is generally held that this is the case, 

 a view which is based upon the fact that the volume of 

 oxygen exhaled is approximately equal to the volume of 

 carbon dioxide absorbed, the process being roughly repre- 

 sented by the equation 



6 CO 2 + s H 2 O = C 6 H 10 O 5 + 6 O,. 



Let us now submit this view to a critical examination. The 

 fact that the volume of the exhaled oxygen is approxi- 

 mately equal to that of the absorbed carbon dioxide may 

 be accepted as established. It is true that, as mentioned 

 above, de Saussure found the volume of the exhaled oxygen 

 to be considerably less than that of the absorbed carbon 

 dioxide, but it is probable that his method of observation 

 may have been imperfect. Boussingault, who subsequently 

 investigated the subject, obtained very different results. In 

 fifteen out of his forty-one experiments, the volume of oxygen 

 exhaled was perceptibly greater than that of the carbon 

 dioxide absorbed; in thirteen the volumes were approximately 

 equal ; in the remainder the volume of oxygen was perceptibly 

 smaller. Taking all the forty-one experiments together, Bous- 

 singault found that for every 100 volumes of carbon dioxide 

 absorbed, 9875 volumes of oxygen had been set free. Further, 

 it has been found by a number of other observers, such as 



